This proposal is designed to test the hypothesis that there are no differences in the intake of essential fatty acids (EFAS) among groups of children with PKU fed different medical foods and normal children, and there are no differences in the plasma and erythrocyte fatty acids amound groups of children undergoing therapy for PKU with different medical foods and normal children. The investigators will compare plasma and EFA intake of children undergoing therapy for PKU who are ingesting Lofenalac, Phenex, Phenyl-free or XP Maximaid to intakes of their normal siblings.